Abstract

Recently, the role of luminance contrast on the perception of velocity has been emphasized by
the so called “footstep illusion” [1]: when a grey bar drifts steadily across a background of
stationary black and white stripes, it appears to stop and start as its contrast to the background
changes. When two bars of unequal grey values drift in parallel, an impression of alternating
footsteps of a moving person can be elicited. This change in apparent speed has been attributed
to the different instantaneous contrasts of the grey bar against the white and black areas of the
background (Contrast Ratio explanation, [1]). We report a series of experiments with further
variants of the footstep illusion to test the validity of the Contrast Ratio (CR) explanation. In
an adjustment task, we presented a standard footstep illusion together with second comparison
stimulus. A total of five participants were required to adjust the contrast of the background
of the comparison stimulus in order to match the strength of the standard illusion. The moving
bars were presented in six luminance levels in the range between 6 and 103 cd/m2 . For
each luminance level we computed the CR by the Weber fraction, following the original study.
We compared the CR values of the standard and comparison stimulus and expected to obtain
comparable CRs within matched illusion pairs. In the first experiment we found a discrepancy
between predicted and obtained CR values. The strength of the illusion was matched at an
increased CR, i.e. participants set the background stripes to higher contrast in order to obtain
the same illusion. In a second experiment the moving bars were rendered transparent, with
a transparency of 50, and we obtained basically the same results as in the first experiment.
In the last experiment we tested whether the footstep illusion can be elicited with isoluminant
red/green patterns. Individual isoluminance levels were identified with flickering stimuli. Subjects
rated an apparent velocity change of the isoluminant moving bars again by adjusting the
contrast of the standard luminance version. The results show clearly that the footstep illusion
occurs also with isoluminant patterns. Furthermore, we found that the adjusted CR was always
near the maximal possible value, suggesting that participants matched the isoluminant illusion
to the strongest available luminance illusion. The present data suggest that foreground and
background stimuli contribute differently to the footstep illusion, a finding which is not predicted
by the current Contrast Ratio account. Most notably, the existence of the illusion with
isoluminant stimuli shows the clearest limitation of the Contrast Ration explanation. That the
strength of the isoluminant footstep illusion was comparable to the luminance version, suggests
a crucial role of chromatic input in an adequate explanation of this phenomenon.